Population structure and conservation biology of the endangered fern Trichomanes speciosum Willd. (Hymenophyllaceae) at its northern distributional limit

Genetic diversity in the Killarney fern, Trichomanes speciosum Willd. has been investigated in south‐western Scotland, the northern‐most limit of the distribution of the sporophyte. T. speciosum is unique amongst European pteridophytes in that both phases of the life cycle are perennial and capable of vegetative propagation. Within sites no variation was revealed by allozyme electrophoresis, even where both generations were growing together. In contrast, diversity was observed among sites, with seven different multilocus phenotypes (MLPs) present in the area. Two of these MLPs covered large areas while the others were restricted to one, or few localities. Asexual reproduction of the gametophyte via gemmae is assumed to be the main means of dispersal in recent times, allowing single clones to become widespread, while the overall genetic variability may be attributed to sexual reproduction and spore dispersal in historic times under more favourable climatic conditions. We suggest that it is not inbreeding, nor lack of genetic variation that limits sporophyte production, but rather the prevailing climatic conditions. The sporophyte is extremely rare and vulnerable. However, when the gametophyte is considered, the species is neither threatened with extinction, nor does it appear to face the danger of marked genetic erosion, because the long‐lived gametophyte stage contains all of the genetic variability present in the area and can be regarded as a valuable ‘seed‐bank’.